Since the 1970s, the graphic size in an integrated circuit has been continuously reduced in the semiconductor industry according to Moore's law, thereby increasing the number of transistors on a central processing unit (CPU) of a computer at the rate of doubling every two years. As the next generation of advanced lithography technology, 22 nm node-oriented extreme ultraviolet lithography opens up a new way with faster speed, smaller size and lower price for the semiconductor industry. However, through the difficult advancement of the EUV (extreme ultraviolet) lithography technology, we can appreciate that the development of the lithography technology is not only based on lithography machines, and the EUV can be just put into mass production as soon as possible through mutual coordination and optimization of other links, for example, appropriate photoresists, defect-free masks and the like are needed. At present, one of main bottlenecks in the development of the extreme ultraviolet lithography is lack of an imaging and detection technology for the masks to ensure the defect-free requirements of the extreme ultraviolet lithography masks.
As any substance has absorption limitation against working wavelength (13.5 nm), if transmission exposure is adopted, the mask can absorb EUV light and the light intensity is greatly reduced. Therefore, in comparison with an existing projection type optical system, the EUV mask adopts a reflection technology rather than a transmission technology. Generally, the manufacturing of the EUV mask adopts a multi-layer stacked Mo/Si film, each of Mo layers and Si layers must be smooth enough and the error allowable range is the size of only one atom. Even dust particles with the size of 10 nm fall on the surface of the mask, serious defects on all samples formed by lithography may be caused. On the standard six-inch (152.4 mm×152.4 mm) mask, such small defects may damage the whole mask and the lithography results. It becomes a crux how to obtain a defect-free multi-layer anti-reflection film on the surface of the mask. Furthermore, very small bulges or depressions on a substrate may also cause the changes in reflection light phase after being covered by the multi-layer film. Such phase type defects may only be about 1 nm in size, so that the phase type defects are almost impossible to be detected by other detection methods in addition to an actual at-wavelength inspection technology. The defects of the extreme ultraviolet lithography mask may appear to have great differences under different detection light sources; if the defects are amplitude type defects, the defects are very small and the wavelength of the required detection light source needs to be smaller than the defects; and if the defects are the phase type defects, in the actual application, the defects in such type are only sensitive to extreme ultraviolet wave bands. Therefore, researchers need to design a special detection system to detect the different types of mask defects. At present, on the road of extreme ultraviolet lithography commercialization, a high-speed and high-resolution mask defect detection and imaging system is essential to ensure the defect-free masks.
However, most of imaging lens used by a currently developed extreme ultraviolet lithography mask detection system use a Schwarzchild objective, which has great processing difficulty, high cost and large volume, so that the difficulty of implementation of the extreme ultraviolet lithography mask detection system is increased.